4K television is beautiful. While shopping for my new 2K (HDTV) set, I happened upon a Sony setup. And this was in Best Buy, not even the kind of specialty store I had to visit for my first two HDTVs (the DLP that just died was a 71"er.. that was pretty exotic 7-8 years ago).
The problem is this... when I bought my first CRT-projection HDTV, it was essentially just an analog HD monitor. There was no content. The specs for over-the-air broadcast hadn't been settled quite. Neither had the video links. So by the time time HD caught on, that first 500lbs TV was already a dinosaur.
When I bought the second, I could compare Blu-ray output with the other types of the day (plasma, LCOS)... so while much has changed even since then, it was still commercially viable technology. Plus, before the second HDTV, I had to get video cameras, authoring hardware, disc formats, etc. in place.
Now it's 4K, and that beautiful display was being driven in-store from an in-store media server, presumably over HDMI 1.4, which only allows quadHD 3840x2160p30 or 4096x2160p24. The HDMI 2.0 spec, which is still a committee not an actual thing, will be needed for 4096x2160p60. So you're likely SOL with today's hardware, already.
And then there are formats. Even given Netflix's crappy HDTV quality, they're not going to have a viable 4K video product using H.264 and even typical networks five years from now. That's just going to get more ISPs adding data caps. 4K is going to push the need for H.265, and perhaps other standards. All the stuff that's coming out now can be expected to go pretty obsolete pretty fast -- that usual early adopter's dilemma. I decided this time to wait awhile... the new LCD-HD is going to be dandy for another 5-10 years....
And really, I need to save my coin for a 4K camcorder anyway... well, there is that $5K model from JVC that came out last year. Or maybe someone will just hack it into a Canon HDSLR...

Anyone thought about video game? That is another big factor pushing people to buy bigger TVs. But to support 4K native resolution by game console, it requires a lot more upgrade, both hardware and software wise. I am sure MS or Sony will be happy to see the adoption of 4K TV but the whole package of 4K experience may be very expensive in the next few years. It is a chicken and egg dilemma again. Until the package cost is on par with regular 1080P TV,4K TV will not thrive IMHO.

There is hope for 4K TV (in contrast to 3D TV) if walking past a 4K TV can cause a consumer to be impressed by the quality. No special glasses, no special position, no special instructions, just an impressive image. The big issues then become the pricing and displaying legacy (standard format) programming whose blurry images just scream at the viewer.

The 4K sets being sold today (except for the Seiki) cost an order of magnitude more than the 1080p sets. That alone could account for a lot of the difference. Couple that with content that's very lightly compressed to show off 4K in the best light...
A more realistic test would to be broadcast a good quality 1080p signal, from a Blu Ray let's say, into two identical 4K sets sitting side by side and letting it one upscale it and one run at 1080p (every 1080p pixel as a 2x2 block of 4K pixels)
That's how almost every 4K set will be used in the next few years, and even after that there probably won't be all that much 4K content. Maybe one HBO and Showtime channel, possibly ESPN for broadcast. If it doesn't noticeably improve the experience via upscaling, it isn't worth the money until the cost premium is almost nil.

The price of the set and the availability of content are critical to most consumers to consider upgrading their TV set.
To content providers - movie makers, the ease of deliver might become the top consideration. UHDTV is aimed at delivering in theaters. It means printing hardcopy (e.g. Bluray) will be as simple as writing the 4k content to the disc. Logistically speaking, a couple steps are eliminated. Time to market will be shorten.
On the other hands, streaming a 4k video might be a challenge and yet, ISP will enjoy the challenge and the thriving of the business.
I agree with Junko that not every family needs a 4K TV. Similarly, not everyone can "afford" to stream 4K. For 4K to thrive, I believe backward compatibility shall be available to viewers. For example, bluray disc player shall be able to downconvert the 4K video to 1080. Similarly, streaming video provider shall allow user to stream 1080 content.

I would say that the biggest difference between 3D TV and UHDTV is that with UHDTV, the consumer doesn't have to do anything different or act any different. It's a passive upgrade.
I think it also falls into the bigger number syndrome. It's pretty easy to promote something based on having an easy to define "bigger number." It's easy for the TV marketers to suggest value and its easy for the purchase to feel value (whether it's real in practice or not)
Other than a cost curve, I can't really see any negatives with UHDTV either. 3D had headaches and limiting viewers to the number of available glasses as clear negatives. UHDTV doesn't have that. My assessment is that there will be an adoption curve similar to regular HDTV, but faster.

Junko, I think you're onto something with your "might win by coincidence" comment. As you said, many older HDTV sets in that 7-10 year age range will soon be replaced. It's not necessarily that they no longer work -- there is also the phenomenon of moving the older, less feature-rich but still working TV to a bedroom or a kid's room and buying a fancy new TV for the main viewing room. A lot of those older HDTV might not do 1080p or aren't "smart" or connected, but are still useful as secondary TV sets.
So when a consumer is thinking about upgrading the main TV set and shuffling the old one off to another room, he or she will soon be faced with the option to go with UHDTV instead of settling for "only 1080p." The difference in quality might be subtle to some viewers, but not so for many others. The sales & marketing machine will emphasize the picture improvement and use words like "future-proof", and if the price premium for UHDTV vs. 1080p is not too stratospheric, the sales volumes should ramp rather quickly -- assuming that UHDTV content availability also ramps quickly, as I believe it will.